Scanning a band of frequencies using an array of high temperature superconductor sensors tuned to the same frequency

ABSTRACT

The methods of the invention for scanning a band of frequencies using a nuclear quadrupole resonance detection system with an array of high temperature superconductor sensors to detect nuclear quadrupole resonance signals improve the nuclear quadrupole resonance detection system performance.

This application claims the benefit of U.S. Application No. 60/566,953,filed on Apr. 30, 2004, which is incorporated in its entirety as a parthereof for all purposes.

FIELD OF THE INVENTION

This invention relates to methods and apparatus for scanning a band offrequencies using an array of high temperature superconductor sensors ina nuclear quadrupole resonance detection system.

BACKGROUND OF THE INVENTION

The use of nuclear quadrupole resonance (NQR) as a means of detectingexplosives and other contraband has been recognized for some time. Seee.g. T. Hirshfield et al, J. Molec. Struct. 58, 63 (1980), A. N.Garroway et al, Proc. SPIE 2092, 318 (1993), and A. N. Garroway et al,IEEE Trans. on Geoscience and Remote Sensing 39, 1108 (2001). NQRprovides some distinct advantages over other detection methods. NQRrequires no external magnet such as required by nuclear magneticresonance. NQR is sensitive to the compounds of interest, i.e. there isa specificity of the NQR frequencies.

A detection system can have one or more coils that both transmit andreceive, or it can have separate coils that only transmit and onlyreceive. A transmit, or transmit and receive, coil of an NQR detectionsystem provides a radio frequency (RF) magnetic field that excites thequadrupole nuclei in the sample, and results in the production of theircharacteristic resonance signals that the receive, or transmit andreceive, coil (i.e. the sensor) detects. The NQR signals have lowintensity and short duration. The transmit, receive, or transmit andreceive, coil preferably has a high quality factor (Q). The transmit,receive, or transmit and receive, coil has typically been a copper coiland therefore has a Q of about 10². It can be advantageous to use atransmit, receive, or transmit and receive, coil made of a hightemperature superconductor (HTS) rather than copper since the HTSself-resonant coil has a Q of the order of 10⁴-10⁶.

The large Q of the HTS self-resonant coil plays an important role duringreception. In view of the low intensity NQR signal, it is important tohave a signal-to-noise ratio (S/N) as large as possible. Thesignal-to-noise ratio is proportional to the square root of Q so thatthe use of the HTS self-resonant coil results in an increase in S/N by afactor of 10-100 over that of the copper system. The use of a hightemperature superconductor sensor, i.e. receive coil, provides adistinct advantage over the use of an ordinary conductor sensor. The HTSsensor can provide greater sensitivity and/or smaller size.

The performance of a signal detection system can be improved by using anarray of sensors, i.e. receive coils, to scan the band of frequencies ofinterest as compared to using a single sensor. An object of the presentinvention is to provide method and apparatus for scanning a band offrequencies using an array of sensors in a nuclear quadrupole resonancedetection system.

SUMMARY OF THE INVENTION

One embodiment of this invention is a method for scanning a band offrequencies ΔF with a nuclear quadrupole resonance detection systemcomprising an array of n high temperature superconductor sensors todetect nuclear quadrupole resonance signals, wherein n≧2 and thebandwidth of each sensor is Δf, by

-   -   (a) determining r different frequencies that span the band of        frequencies ΔF when using sensors with bandwidths Δf, wherein r        is of the order of ΔF/Δf;    -   (b) tuning the resonance frequencies of the n sensors to n        different resonance frequencies, wherein the n different        resonance frequencies are selected from the group of the r        different frequencies, and maintaining these resonance        frequencies for a selected period of time;    -   (c) retuning simultaneously the resonance frequencies of the n        sensors to n different frequencies selected from the group of        the r different frequencies, and maintaining these retuned        resonance frequencies for a selected period of time, wherein        each of the n sensors has a retuned resonance frequency that is        different from the resonance frequency to which it was tuned or        retuned in any previous step; and    -   (d) repeating step (c) r-2 times wherein the resonance frequency        of each sensor is tuned for one period of time to each of the r        different frequencies, and no two sensors are tuned to the same        resonance frequency at the same time.

Steps (b), (c) and (d) may be repeated one or more times, the signalsdetected by all sensors that are tuned to a selected frequency may becombined, and the periods of time for which the resonance frequenciesare maintained in steps (b), (c) and (d) may be the same.

Another embodiment of this invention is a method for scanning a band offrequencies ΔF with a nuclear quadrupole resonance detection systemcomprising an array of n high temperature superconductor sensors todetect nuclear quadrupole resonance signals, wherein n≧4 and thebandwidth of each sensor is Δf, by

-   -   (a) dividing the sensors into m groups with p sensors in each        group, determining r different frequencies that span the band of        frequencies ΔF when using sensors with bandwidths Δf, wherein r        is of the order of ΔF/Δf, dividing the r different frequencies        into s sets with p of the r different frequencies in each set,        and assigning each set of frequencies to one of the groups of        sensors;    -   (b) tuning the resonance frequencies of the p sensors in each        group to the p different frequencies in the set of frequencies        assigned to that group, and maintaining these tuned resonance        frequencies for a selected period of time;    -   (c) retuning simultaneously the resonance frequencies of the p        sensors in each group to p different frequencies within the same        set assigned to that group for step (b), and maintaining these        retuned resonance frequencies for a selected period of time,        wherein each of the p sensors has a retuned resonance frequency        that is different from the resonance frequency to which it was        tuned or retuned in any previous step;    -   (d) repeating step (c) p-2 times;    -   (e) retuning the resonance frequencies of the p sensors in each        group to the p different frequencies in a second set of        frequencies assigned to that group, and maintaining these        retuned resonance frequencies for a selected period of time;    -   (f) retuning simultaneously the resonance frequencies of the p        sensors in each group to p different frequencies within the same        set assigned to that group for step (e), and maintaining these        retuned resonance frequencies for a selected period of time,        wherein each of the p sensors has a retuned resonance frequency        that is different from the resonance frequency to which it was        tuned or retuned in any previous step;    -   (g) repeating step (f) p-2 times; and    -   (h) repeating steps (e), (f) and (g) for each additional set of        p different frequencies until all s sets of frequencies have        been used as resonance frequencies for one of the m groups of        sensors.

Steps (b) through (h) may be repeated one or more times, the signalsdetected by all sensors that are tuned to a selected frequency may becombined, and the periods of time for which the resonance frequenciesare maintained in steps (b) through (h) may be the same.

A further embodiment of this invention is a method for scanning a bandof frequencies ΔF with a nuclear quadrupole resonance detection systemcomprising an array of n high temperature superconductor sensors todetect nuclear quadrupole resonance signals, wherein n≧4 and thebandwidth of each sensor is Δf, by

-   -   (a) dividing the n sensors into m groups with p sensors in each        group, determining r different frequencies that span the band of        frequencies ΔF when using sensors with bandwidths Δf, wherein r        is of the order of ΔF/Δf, dividing the r different frequencies        into m sets with at least p of the r different frequencies in        each set, and assigning one set of frequencies to each group of        sensors;    -   (b) tuning the resonance frequencies of the p sensors in each        group to p different frequencies selected from any of the        frequencies assigned to that group, and maintaining these tuned        resonance frequencies for a selected period of time;    -   (c) retuning simultaneously the resonance frequencies of the p        sensors in each group to p different frequencies selected from        any of frequencies assigned to that group, and maintaining these        retuned resonance frequencies for a selected period of time,        wherein each of the p sensors has a retuned resonance frequency        that is different from the resonance frequency to which it was        tuned or retuned in any previous step; and    -   (d) repeating step (c) until all of the frequencies in the sets        of frequencies assigned to each group have been used as a        resonance frequency by each sensor in the group.

Steps (b), (c) and (d) may be repeated one or more times, the signalsdetected by all sensors that are tuned to a selected frequency may becombined, and the periods of time for which the resonance frequenciesare maintained in steps (b), (c) and (d) may be the same.

Yet another embodiment of this invention is a nuclear quadrupoleresonance detection system for scanning a sample, comprising

-   -   (a) n high temperature superconductor sensors, each with        bandwidth Δf, to scan a band of frequencies ΔF and detect any        nuclear quadrupole resonance signal within the band of        frequencies ΔF, wherein n≧2;    -   (b) means to tune the resonance frequencies of the n sensors to        n different frequencies, wherein the n different frequencies are        selected from the group of r different frequencies that span the        band of frequencies ΔF using sensors with bandwidths Δf, and        wherein r is of the order of ΔF/Δf; and    -   (c) means to simultaneously retune r-1 times the resonance        frequencies of the n sensors to n different frequencies each        time, wherein after each retuning, each of the n sensors has a        retuned resonance frequency that is different from any of the        resonance frequencies to which it was previously tuned or        retuned.

Yet another embodiment of this invention is a method for scanning a bandof frequencies ΔF with a nuclear quadrupole resonance detection systemcomprising an array of n high temperature superconductor sensors todetect nuclear quadrupole resonance signals, wherein n≧2 and thebandwidth of each sensor is Δf, by

-   -   (a) determining r different frequencies that span the band of        frequencies ΔF when using sensors with bandwidths Δf, wherein r        is of the order of ΔF/Δf;    -   (b) tuning the resonance frequencies of the n sensors to n        different frequencies, wherein the n different frequencies are        selected from the group of the r different frequencies, and        maintaining the resonance frequencies of the n sensors for a        selected period of time;    -   (c) retuning the resonance frequencies of some or all of the n        sensors to any of the r different frequencies that were not used        in any previous step, wherein the n sensors have n different        resonance frequencies, and maintaining the resonance frequencies        of the n sensors for a selected period of time; and    -   (d) repeating step (c) until all the r different frequencies        have been used as resonance frequencies.

Steps (b), (c) and (d) may be repeated one or more times, the signalsdetected by all sensors that are tuned to a selected frequency may becombined, the periods of time for which the resonance frequencies aremaintained in steps (b), (c) and (d) may be the same, and in step (c)the resonance frequencies of all of the n sensors are retuned to any ofthe r different frequencies that were not used in any previous step.

Yet another embodiment of this invention is a nuclear quadrupoleresonance detection system for scanning a sample, comprising:

-   -   (a) n high temperature superconductor sensors, each with        bandwidth Δf, to scan a band of frequencies ΔF and detect any        nuclear quadrupole resonance signal within the band of        frequencies ΔF, wherein n≧2;    -   (b) means to tune the resonance frequencies of the n sensors to        n different frequencies, wherein the n different frequencies are        selected from the group of r different frequencies that span the        band of frequencies ΔF using sensors with bandwidths Δf, and        wherein r is of the order of ΔF/Δf; and    -   (c) means to simultaneously retune the resonance frequencies of        the n sensors to n different frequencies that are not any of the        r different frequencies used previously as resonance        frequencies, and to continue to retune the n sensors in the same        manner until all the r different frequencies have been used as        resonance frequencies.

Yet another embodiment of this invention is a nuclear quadrupoleresonance detection system for scanning a sample, comprising:

-   -   (a) n high temperature superconductor sensors, each with        bandwidth Δf, to scan a band of frequencies ΔF and detect any        nuclear quadrupole resonance signal within the band of        frequencies ΔF, wherein n≧2; and    -   (b) means to tune the resonance frequencies of each of the n        sensors to r different frequencies, wherein the r different        frequencies span the band of frequencies ΔF using sensors with        bandwidths Δf and wherein r is of the order of ΔF/Δf.

Yet another embodiment of this invention is a method for scanning a bandof frequencies ΔF with a nuclear quadrupole resonance detection systemcomprising an array of n high temperature superconductor sensors todetect nuclear quadrupole resonance signals, wherein n≧2 and thebandwidth of each sensor is Δf, by

-   -   (a) determining r different frequencies that span the band of        frequencies ΔF when using sensors with bandwidths Δf, wherein r        is the order of ΔF/Δf;    -   (b) tuning the resonance frequencies of all the n sensors to the        same frequency, wherein the frequency is selected from the group        of the r different frequencies, and maintaining the resonance        frequencies for a selected period of time;    -   (c) retuning simultaneously the resonance frequencies of all of        the n sensors to the same frequency, wherein the retuned        frequency is another of the r different frequencies, and        maintaining these retuned resonance frequencies for a selected        period of time; and    -   (d) repeating step (c) r-2 times, wherein after each retuning        the retuned resonance frequencies of the n sensors are different        from the resonance frequencies to which the sensors are tuned or        retuned in any of the previous steps.

Steps (b), (c) and (d) may be repeated one or more times, the signalsdetected by all sensors that are tuned to a selected frequency may becombined, and the periods of time for which the resonance frequenciesare maintained in steps (b), (c) and (d) may be the same.

Yet another embodiment of this invention is a nuclear quadrupoleresonance detection system for scanning a sample, comprising:

-   -   (a) n high temperature superconductor sensors, each with        bandwidth Δf, to scan a band of frequencies ΔF and detect any        nuclear quadrupole resonance signal within the band of        frequencies ΔF, wherein n≧2;    -   (b) means to tune the resonance frequencies of all the n sensors        to the same resonance frequency, wherein the resonance frequency        is selected from the group of r different frequencies that span        the band of frequencies ΔF using sensors with bandwidths Δf, and        wherein r is of the order of ΔF/Δf; and    -   (c) means to simultaneously retune r-1 times the resonance        frequencies of all the n sensors to the same frequency each        time, wherein the frequency each time is another of the r        different frequencies, and wherein after each retuning each of        the n sensors has a retuned resonance frequency that is        different from any resonance frequencies to which it has        previously been retuned.

The methods of the invention for scanning a band of frequencies in anuclear quadrupole resonance detection system are especially useful whenthe nuclear quadrupole resonance detection system is being used fordetecting the nuclear quadrupole resonance of explosives, drugs andother contraband.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one embodiment for scanning a band of frequencies using anarray of four sensors wherein all four sensors have the same resonancefrequency at any given time and are stepped in unison through thefrequency band.

FIG. 2 shows a second embodiment for scanning a band of frequenciesusing an array of four sensors wherein the resonance frequencies areinitially set at four different frequencies and kept fixed at thosefrequencies.

FIG. 3 shows another embodiment of the invention for scanning a band offrequencies using an array of four sensors wherein the resonancefrequencies are initially set at four different frequencies, and theresonance frequencies are then simultaneously retuned to the fourdifferent frequencies at 3 subsequent times, wherein each of the foursensors has a retuned resonance frequency that is different from any ofits previous resonance frequencies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention provides methods for scanning a band of frequencies in aNQR detection system. This invention also provides NQR detection systemsthat can accomplish these improved performances.

The high Q and relatively small size of an HTS sensor makes the use ofan array of such sensors feasible, and the method of scanning this arrayis critical to the performance of the NQR detection system using thearray. There are various reasons that would make scanning a band offrequencies desirable in a NQR detection system. One motivation for suchscanning arises from the temperature dependence of NQR frequencies. Thetemperature of the sample to be scanned as a source of NQR may be knownonly within some range of temperature. As a result, the NQR frequency isknown only within a range of frequencies, i.e. a band of frequencies. Todetect the NQR signal there must be detection capability over the bandof frequencies. Another reason for scanning a band of frequencies is tosearch for more than one NQR frequency.

One method to carry out such a scan comprises tuning all the sensors inthe array to the same frequency, a selected frequency within the band,and then frequency stepping all of the sensors in the array, i.e.changing the resonance frequencies of all the sensors in the array inthe same manner. This method is illustrated in FIG. 1 with an array offour sensors. For simplicity, four frequency settings f₁, f₂, f₃ and f₄are assumed to span the frequency band of interest. FIG. 1 illustratesthe method and shows the resonance frequencies of the four sensors atfour different times t₁, t₂, t₃ and t₄. Since the resonance frequenciesof the sensors are stepped through the band of frequencies in the sameway, all four sensors have the same resonance frequency at any giventime.

As described above, it is advantageous for sensors to have the high Q ofthe high temperature superconductor sensors used in this invention. Withthe high Q, there is a corresponding narrow bandpass, i.e. the bandwidthof the sensor Δf=f_(R)/Q, where f_(R) is the resonance frequency. Thenumber of different frequencies r that span the band of frequencies ΔFwhen using sensors with bandwidths Δf is of the order of ΔF/Δf. Theratio ΔF/Δf=w, where w is the minimum number of frequencies spanning theband of frequencies ΔF when scanning with sensors of bandwidths Δf, i.e.to span the band of frequencies, the sensors must be tuned to at least wdifferent frequencies. It may be advantageous to increase the number offrequencies used to increase the sensitivity of the measurements. Asused herein, “the r different frequencies that span the band offrequencies ΔF when using sensors with bandwidths Δf, wherein r is ofthe order of ΔF/Δf” means that r is at least equal to w, and can be upto three times w.

In view of the narrow bandpass Δf and depending on n, the number ofsensors in the array, and the width ΔF of the band of frequencies to bescanned, r may be greater than n. This method of scanning then comprisesinitially tuning all n sensors to the same frequency where thatfrequency is selected from the group of r different frequencies. Theresonance frequencies of all n sensors are then simultaneously retunedr-1 times, wherein the resonance frequency of all the sensors each timeis another of the r different frequencies, and wherein after eachretuning the retuned resonance frequencies of the n sensors aredifferent from the resonance frequencies used in any of the previousretunings. Upon completion of the r-1 retunings, all r differentfrequencies have been used as resonance frequencies. The r differentfrequencies do not have to be used as resonance frequencies in anyparticular order. While the sensors will typically be made asessentially identical as possible, there will be some small variation inproperties from sensor to sensor, including in Δf.

In many NQR detection systems, the sensors will be closely spaced. Thereis a coupling between these sensors as a result of the mutual inductancebetween them. This coupling between two sensors increases as thedistance between them decreases. The problem of the coupling betweensensors is exacerbated when using HTS sensors because of the very largeQ's of the HTS sensors. This coupling of sensors can have seriouseffects on the performance of the NQR detection system. When the sensorsare so closely spaced that coupling interferes with detection systemperformance, it is preferred to employ a device to decouple the sensors.If a device to decouple the sensors is not employed, however, it wouldbe preferable to use one of the methods of the invention for scanning aband of frequencies described below instead of the method describedabove in which all the n sensors have the same resonance frequency.

One such alternative method to scan a band of frequencies of interestcomprises having n sensors in the array with each having a differentresonance frequency. The method is illustrated in FIG. 2. For simplicityand comparison, an array of four sensors and the same four frequenciesf₁, f₂, f₃ and f₄ used in FIG. 1 are used again in FIG. 2. The fourfrequencies are again assumed to span the frequency band of interest.FIG. 2 shows the resonance frequencies of the four sensors at fourdifferent times t₁, t₂, t₃ and t₄. The resonance frequencies areinitially set at four different frequencies and kept fixed at thosefrequencies. For those situations, in which r is greater than n asdiscussed above, the resonance frequencies of the n sensors are firsttuned to n different frequencies selected from the group of r differentfrequencies. The resonance frequencies of some or all of the n sensorsare then retuned to any of the r different frequencies that were notused previously. Preferably, all of the n sensors are retuned. Theretuning is continued until all r frequencies are used as resonancefrequencies. Coupling can be further reduced by choosing the n differentfrequencies, and the sensors with these resonance frequencies, suchthat, to the extent possible, frequencies are selected that are not tooclose in frequency and the closer frequencies are spatially separated.While coupling can be reduced by this method, combining the signals fromdifferent sensors to achieve noise reduction may have reducedeffectiveness, and the spatial sensitivity of the array is limited.

In another embodiment that provides excellent spatial sensitivity andreduced coupling, the method comprises tuning the resonance frequenciesof the n high temperature superconductor sensors to n differentfrequencies, and retuning the resonance frequencies of all of thesensors r-1 times. The resonance frequencies of the n sensors are alwaysdifferent, and the same resonance frequency is used only once for eachsensor. At the conclusion of the r-1 retunings, all of the r differentfrequencies will have been sued as resonance frequencies for each of then sensors. This method is illustrated in FIG. 3 using the same array offour sensors and four frequencies used in FIGS. 1 and 2. FIG. 3 showsthe resonance frequencies of the four sensors at four different timest₁, t₂, t₃ and t₄. The resonance frequencies of the four sensors areinitially set at four different frequencies f₁, f₂, f₃ and f₄. Thesefour frequencies are assumed to span the frequency band of interest. Theresonance frequencies are maintained for a selected period of time andare then simultaneously retuned to the four different frequencies. Thissimultaneous retuning is repeated two more times with the proviso thateach of the four sensors has a retuned resonance frequency that isdifferent from any of its previous resonance frequencies. At thecompletion of the retunings, each of the sensors has had its resonancefrequency set at each of the four frequencies, and each of thefrequencies was maintained for the selected period of time. The fourtimes t₁, t₂, t₃ and t₄ represent a time in each of the four given timeperiods. The four frequencies are shown rotated counter-clockwise amongthe four sensors in FIG. 3, but various assignments of frequencies canbe used.

When this method is applied in those situations in which r is greaterthan n as discussed above, the resonance frequencies of the n sensorsare first tuned to n different frequencies selected from the group of rdifferent frequencies. The resonance frequencies of the n sensors arethen simultaneously retuned to n different frequencies selected from thegroup of r different frequencies. Each of the n sensors has a retunedresonance frequency that is different from its previously tuned orretuned resonance frequency. The retuning step is repeated r-2 times,wherein each of the n sensors has a retuned resonance frequency that isdifferent from its resonance frequency in any of the previous steps andwherein some or all of the n frequencies selected in any retuning stepmay be different from the n frequencies selected for the previoustuning. It is desirable to have those sensors with nearest resonancefrequencies spatially separated. This is more readily accomplished asthe number of sensors in the array increases. This method of theinvention further comprises combining the signals detected by the nsensors when their resonance frequencies were tuned to the samefrequency to thereby improve the signal-to-noise ratio.

In another embodiment, the method comprises using an array of n hightemperature superconductor sensors wherein n≧4. The method comprisesdividing the n sensors into m groups with p sensors in each group,dividing the r different frequencies that span the band of frequenciesinto s sets with p of the r different frequencies in each set, andassigning each set of frequencies to one of the groups of sensors. Inthe typical situation in which this method would be employed, r would begreater than n so that s would be greater than m, and some or all of them groups of sensors would be assigned two or more of the s sets offrequencies. The resonance frequencies of the p sensors in each groupare tuned to the p different frequencies in a set assigned to thatgroup. The resonance frequencies of the p sensors in each group are thensimultaneously retuned to p different frequencies within the same setpreviously assigned to that group. Each of the p sensors has a retunedresonance frequency that is different from the resonance frequency towhich it was previously tuned or retuned. The retuning step is repeatedp-2 times, wherein each of the p sensors in each group has a retunedresonance frequency that is different from the resonance frequency usedfor it in any of the previous steps.

The resonance frequencies of the p sensors in each group are thensimultaneously retuned to p different frequencies in a second set offrequencies assigned to that group. The resonance frequencies of the psensors in each group are then simultaneously retuned to p differentfrequencies in the same second set of frequencies used previously forthat group. Each of the p sensors has a retuned resonance frequency thatis different from any resonance frequency to which it was previouslytuned or retuned. The retuning step is repeated p-2 times, wherein eachof the p sensors in each group has a retuned resonance frequency that isdifferent from the resonance frequency used for it in any of theprevious steps. Retuning is continued until all s sets of frequencieshave been used as resonance frequencies by one of the m groups ofsensors. This method further comprises combining the signals detected bythe p sensors in each group when their resonance frequencies were tunedto the same frequency to thereby improve the signal-to-noise ratio.

Alternatively, the r different frequencies are divided into m sets withat least p of the r different frequencies in each set. One set offrequencies is assigned to each group of sensors. The resonancefrequencies of the p sensors in each group are tuned to p differentfrequencies selected from any of the frequencies assigned to that group.The resonance frequencies of the p sensors in each group are retuned top different frequencies selected from any of frequencies assigned tothat group. Each of the p sensors has a retuned resonance frequency thatis different from the resonance frequency used for it in any previousstep. The retuning step is repeated until all of the frequencies in thesets of frequencies assigned to each group have been used as a resonancefrequency by each sensor in the group.

Tuning or retuning the resonance frequency of a sensor may beaccomplished in a variety of ways. Means for tuning and/or retuning theresonance frequency of a sensor may include, for example, using two ormore movable, coupled high temperature superconductor self-resonantcoils. The resonance frequency of the fundamental symmetric mode of thetwo or more coupled high temperature superconductor self-resonant coilscan then be varied by mechanically displacing one or more coils withrespect to the others, and these coupled coils as mechanically adjustedwill serve as the HTS sensor coil. Preferably, the two or more coils areplanar, i.e. surface, coils. Each planar coil can have a HTS coilconfiguration on only one side of the substrate or essentially identicalHTS coil configurations on both sides of the substrate. Preferably, theHTS sensors are each comprised of a high temperature superconductorself-resonant planar coil or two or more coupled high temperaturesuperconductor self-resonant planar coils.

Alternatively, for a sensor comprised of a high temperaturesuperconductor self-resonant coil or two or more coupled hightemperature superconductor self-resonant coils, means for tuning and/orretuning the resonance frequency of the sensor to a specified frequencymay include a circuit, preferably one for each sensor. The circuit iscomprised of a single loop or coil to inductively couple the circuit tothe high temperature superconductor self-resonant sensor, a reactance inseries with the single loop or coil, and means to connect the reactanceto, and disconnect the reactance from, the single loop or coil. Thesingle loop or coil can be made of a regular conductor such as copper ora high temperature superconductor. The reactance can be an inductance,capacitance or combination of both. Means to connect the reactance to,and disconnect the reactance from, the single loop or coil may includeat least one mechanical switch or electrical switch such as a diode.Preferably, the reactance can be varied so that the resonance frequencycan be adjusted to more than one frequency. A variable reactance may beprovided where the reactance is comprised of two or more capacitors inparallel, each of which can be individually connected to or disconnectedfrom the single loop or coil. Alternatively, a variable reactance may becomprised of two or more inductors in series, each of which can beindividually connected to or disconnected from the single loop or coilby a mechanical or electrical switch that can short-circuit the inductorand thereby essentially remove it from the circuit.

This invention thus provides a nuclear quadrupole resonance detectionsystem for scanning a sample. The detection system is comprised of nhigh temperature superconductor sensors to scan a band of frequenciesand detect any nuclear quadrupole resonance signal within the band offrequencies. The detection system also comprises means to tune and/orretune the resonance frequencies of the n sensors to r differentfrequencies, wherein the r different frequencies span the band offrequencies. As indicated above, means for tuning and/or retuning mayinclude a circuit comprised of a single loop or coil to inductivelycouple the circuit to the high temperature superconductor self-resonantsensor, a reactance in series with the single loop or coil, and means toconnect the reactance to and disconnect the reactance from the singleloop or coil. When two or more coupled high temperature superconductorself-resonant coils are used as a sensor, means for tuning and/orretuning the resonance frequency may include micropositioners tomechanically displace one or more coils with respect to the others. Inaddition, the detection system is comprised of means to simultaneouslyretune the resonance frequencies of the n sensors. Means tosimultaneously retune the n sensors may include those described abovefor the initial tuning.

This nuclear quadrupole resonance detection system further comprisesmeans to combine the signals detected by the n sensors when theirresonance frequencies were tuned to the same frequency to therebyimprove the signal-to-noise ratio. The NQR signals detected by allsensors tuned to a selected resonance frequency can be combined invarious ways to enhance detection system performance. For example, meansto combine signals may include a programmed microprocessor to add thesignals coherently, i.e. the phases of the individual signals areadjusted to add constructively, by various analog and digitaltechniques. In the analog technique for coherent addition, theelectrical path from each sensor to the combination point, at which thesignals are added, is adjusted so that the signals add constructively atthe combination point. When the sensors are essentially equidistant fromthe sample that is the source of the nuclear quadrupole resonancesignal, the electrical paths from the sensors to the combination pointcan be made essentially identical thereby insuring that the signals addconstructively at the combination point. In the digital technique forcoherent addition, each signal detected by the sensors is multiplied,before combination, by a constant complex factor that can be measured orcalculated to correct for phase differences between the signal paths andthereby insure that the signals add constructively at the combinationpoint. The constant complex factor is specific to each electrical pathfrom each sensor to the combination point. Typically, the signals willbe amplified before they are added.

The advantages of combining signals can be seen as follows. The signal Sobtained by coherently adding the signals from n sensors is proportionalto n. Assuming that the noise present is random, the noise N, after thecoherent addition, is proportional to n^(0.5). Therefore S/N isproportional to n^(0.5), i.e. the square root of n. The combination ofsignals from two sensors therefore increases S/N by a factor of 1.4. Thecombination of signals from four sensors increases S/N by a factor of 2.

The sensors used to detect the nuclear quadrupole resonance signals canbe used only as receive coils, or as both transmit and receive coils.Preferably, separate coils are used to transmit the RF signal and todetect any NQR signals, and the sensors are thus used solely as receivecoils, i.e. they solely detect the nuclear quadrupole resonance signalsand are not used to transmit an excitation signal.

The transmit coils used in this invention can be made of normal metalssuch as copper, silver and aluminum, or can be made from a hightemperature superconductor. A normal metal coil is preferably in theform of a shielded-loop resonator (SLR) coil. SLRs have been developedto eliminate the detuning effect of the electrical interaction betweenthe coil and the surrounding material.

Preferably, one or more SLR copper transmit coils are used to apply anRF signal to the sample to be scanned. Preferably, the sensors are hightemperature superconductor (HTS) coils in the form of a self-resonantplanar coil, i.e. a surface coil, with a coil configuration of HTS ononly one side of the substrate or essentially identical HTS coilconfigurations on both sides of the substrate. High temperaturesuperconductors are those that superconduct above 77K. The hightemperature superconductors used to form the HTS self-resonant coil arepreferably selected from the group consisting of YBa₂Cu₃O₇,Tl₂Ba₂CaCu₂O₈, TlBa₂Ca₂Cu₃O₉, (TlPb)Sr₂CaCu₂O₇ and (TlPb)Sr₂Ca₂Cu₃O₉.Most preferably, the high temperature superconductor is YBa₂Cu₃O₇ orTl₂Ba₂CaCu₂O₈.

An HTS self-resonant coil can be formed by various known techniques.Preferably, a planar coil is formed by first depositing HTS layers onboth sides of a single crystal substrate. In a preferred technique forforming a Tl₂Ba₂CaCu₂O₈ coil, the HTS layers are formed directly on asingle crystal LaAlO₃ substrate or on a CeO₂ buffer layer on a singlecrystal sapphire (Al₂O₃) substrate. An amorphous precursor layer ofBa:Ca:Cu oxide about 500 nm thick and with a stoichiometry of about2:1:2 is deposited by off-axis magnetron sputtering from a Ba:Ca:Cuoxide target. The precursor film is then thallinated by annealing it inair for about 45 minutes at 850° C. in the presence of a powder mixtureof Tl₂Ba₂Ca₂Cu₃O₁₀ and Tl₂O₃. When this powder mixture is heated, Tl₂Oevolves from the powder mixture, diffuses to the precursor film andreacts with it to form the Tl₂Ba₂CaCu₂O₈ phase. The sample is thencoated with photoresist on both sides and baked.

A coil design mask is prepared. The design mask is then centered on thephotoresist covering the Tl₂Ba₂CaCu₂O₈ film on the front side of thesubstrate and exposed to ultraviolet light. If the coil is to have thesame HTS pattern on both sides of the substrate, the design mask is thencentered on the photoresist covering the Tl₂Ba₂CaCu₂O₈ film on the backside of the substrate and exposed to ultraviolet light. The resist isthen developed and the portion of the Tl₂Ba₂CaCu₂O₈ film exposed whenthe resist is developed is etched away by argon beam etching. Theremaining photoresist layer is then removed by an oxygen plasma. Theresult is the desired HTS planar coil. If two or more coupled hightemperature superconductor self-resonant coils are to be used as thesensor coil, additional coils can be produced using the same technique.

Provision must be made in the detection system of this invention for apower supply to supply power for transmitting an RF excitation pulse.Provision must also be made for cooling the HTS materials to at leastliquid nitrogen temperature.

An NQR detection system comprised of an array of two or more hightemperature superconductor sensors can be used to detect the presence ofchemical compounds for any purpose, but is particularly useful fordetecting the presence of controlled substances such as explosives,drugs or contraband of any kind. Such an NQR detection system could beusefully incorporated into a safety system, a security system, or a lawenforcement screening system. Such systems would typically include amechanical system for automatically moving objects to be scanned intoand out of the range of the scanner, and may also include an enclosurein which objects to be subjected to NQR scanning could be placed andshielded from the environment during the test. Where such an enclosureis used, the mechanical system would automatically moving objects to bescanned into and out of the enclosure.

Where an apparatus or method of this invention is stated or described ascomprising, including, containing, having, being composed of or beingconstituted by certain components or steps, it is to be understood,unless the statement or description explicitly provides to the contrary,that one or more components or steps other than those explicitly statedor described may be present in the apparatus or method. In analternative embodiment, however, the apparatus or method of thisinvention may be stated or described as consisting essentially ofcertain components or steps, in which embodiment components or stepsthat would materially alter the principle of operation or thedistinguishing characteristics of the apparatus or method would not bepresent therein. In a further alternative embodiment, the apparatus ormethod of this invention may be stated or described as consisting ofcertain components or steps, in which embodiment components or stepsother than those as stated or described would not be present therein.

Where the indefinite article “a” or “an” is used with respect to astatement or description of the presence of a component in an apparatus,or a step in a method, of this invention, it is to be understood, unlessthe statement or description explicitly provides to the contrary, thatthe use of such indefinite article does not limit the presence of thecomponent in the apparatus, or of the step in the method, to one innumber.

1. A method for scanning a band of frequencies ΔF with a nuclearquadrupole resonance detection system comprising an array of n hightemperature superconductor sensors to detect nuclear quadrupoleresonance signals, wherein n≧2 and the bandwidth of each sensor is Δf,comprising: a) determining r different frequencies that span the band offrequencies ΔF when using sensors with bandwidths Δf, wherein r is theorder of ΔF/Δf; b) tuning the resonance frequencies of all the n sensorsto the same frequency, wherein the frequency is selected from the groupof the r different frequencies, and maintaining the resonancefrequencies for a selected period of time; c) retuning simultaneouslythe resonance frequencies of all of the n sensors to the same frequency,wherein the retuned frequency is another of the r different frequencies,and maintaining these retuned resonance frequencies for a selectedperiod of time; and d) repeating step (c) r-2 times, wherein after eachretuning the retuned resonance frequencies of the n sensors aredifferent from the resonance frequencies to which the sensors are tunedor retuned in any of the previous steps.
 2. The method of claim 1,further comprising repeating steps (b), (c) and (d) one or more times.3. The method of claim 1, further comprising combining the signalsdetected by all sensors that are tuned to a selected frequency.
 4. Themethod of claim 1, wherein each resonance frequency is maintained forthe same period of time.
 5. The method of claim 1, wherein the n hightemperature superconductor sensors solely detect nuclear quadrupoleresonance signals.
 6. The method of claim 5, wherein each hightemperature superconductor sensor is comprised of a high temperaturesuperconductor self-resonant planar coil.
 7. The method of claim 5,wherein each high temperature superconductor sensor is comprised of twoor more coupled high temperature superconductor self-resonant planarcoils.
 8. A nuclear quadrupole resonance detection system for scanning asample, comprising: a) n high temperature superconductor sensors, eachwith bandwidth Δf, to scan a band of frequencies ΔF and detect anynuclear quadrupole resonance signal within the band of frequencies ΔF,wherein n≧2; b) means to tune the resonance frequencies of all the nsensors to the same resonance frequency, wherein the resonance frequencyis selected from the group of r different frequencies that span the bandof frequencies ΔF using sensors with bandwidths Δf, and wherein r is ofthe order of ΔF/Δf; and c) means to simultaneously retune r-1 times theresonance frequencies of all the n sensors to the same frequency eachtime, wherein the frequency each time is another of the r differentfrequencies, and wherein after each retuning each of the n sensors has aretuned resonance frequency that is different from any resonancefrequencies to which it has previously been retuned.
 9. The nuclearquadrupole resonance detection system of claim 8, further comprising: d)means to combine the signals detected by all sensors that are tuned to aselected frequency.
 10. The nuclear quadrupole resonance detectionsystem of claim 8, wherein the n high temperature superconductor sensorssolely detect nuclear quadrupole resonance signals.
 11. The nuclearquadrupole resonance detection system of claim 10, wherein each hightemperature superconductor sensor is comprised of a high temperaturesuperconductor self-resonant planar coil.
 12. The nuclear quadrupoleresonance detection system of claim 10, wherein each high temperaturesuperconductor sensor is comprised of two or more coupled hightemperature superconductor self-resonant planar coils.
 13. The nuclearquadrupole resonance detection system of claim 10, further comprisingone or more shielded-loop resonator coils to apply to the sample to bescanned for nuclear quadrupole resonance an RF signal capable ofexciting quadrupolar nuclei in the sample.
 14. The nuclear quadrupoleresonance detection system of claim 10, wherein the means to tune andsimultaneously retune the resonance frequency of each of the n sensorsis comprised of n circuits, one for each of the n sensors, wherein eachcircuit is comprised of (a) a single loop or coil to inductively couplethe circuit to the high temperature superconductor self-resonant sensor,(b) a variable reactance in series with the single loop or coil, and (c)means to connect the variable reactance to, and disconnect the variablereactance from, the single loop or coil.
 15. The nuclear quadrupoleresonance detection system of claim 10, wherein the means to tune andsimultaneously retune the resonance frequency of each of the n sensorsis comprised of one or more circuits that is comprised of a variablereactance.
 16. The nuclear quadrupole resonance detection system ofclaim 8, wherein the sample comprises explosives, drugs or othercontraband.
 17. The nuclear quadrupole resonance detection system ofclaim 9, wherein the sample comprises explosives, drugs or othercontraband.
 18. A safety system, a security system, or a law enforcementscreening system comprising the nuclear quadrupole resonance detectionsystem of claim 9.